1. Field of the Invention
The present invention relates to a thermoluminescent dosimeter for radiation monitoring, comprising LiF doped with Mg, Cu and Si, and a fabrication method thereof. Particularly, the present invention relates to a thermoluminescent dosimeter that shows high thermal stability and thus maintains constant sensitivity even upon high-temperature annealing, has a remarkably low residual signal, and can maintain the same sensitivity as its initial readout value even when it is reused.
2. Description of the Prior Art
Various types of thermoluminescent detectors for detecting radiation employ various principles in various application fields. Examples thereof include: gas-filled counters, employing the principle of ionizing gas molecules by radiation; semiconductor detectors, employing the principle by which a semiconductor material produces electron-hole pairs upon exposure to radiation; scintillation counters, employing a material that generates scintillation upon exposure to radiation; film badges, employing the response of photographic films to radiation; and thermoluminescence dosimeters (hereinafter, “TLD”), employing the principle by which a material, such as an insulator, emits light when thermally stimulated after it is irradiated.
Among these radiation detectors, the TLD is a radiation detector which is widely used to measure personal exposures to radiation. In the thermoluminescence process, when an insulator or ionic crystal/crystalline material (solids) is irradiated with radiation (X and gamma rays, beta rays or alpha particles), electrons in the valence band are excited to reach the conduction band a large portion of these electrons will return to the valence band in a very short time, but some of them get located in a trapping energy levels (traps) within the forbidden band (valence band and conduction band are separated by fornidden band) due to the defects in the crystals. When electrons in the trapping level have no thermal disturbance, they may remain in these traps for a considerably long period of time. The number of trapped electrons is dependent upon the amount of ionizing radiation encountered by the solid.
However, when heat energy such as heating is applied to the irradiated solid from an external source, electrons no longer remain in the trapping level and move to the conduction band and recombine with holes at the recombination centre in the forbidden band to emit light of energy corresponding to the dexcitation energy of the recombination. The emission of the light is usually proportional to the amount of radiation dose, and thus can be used to determine the dose of radiation. Herein, the emitted light is used to measure personal exposures of radiation workers or doses encountered in the radiodiagnosis and delivered during radiation therapy treatment of patients.
The performance of thermoluminescent dosimeters comprising thermoluminescent materials depends on the thermoluminescent properties. In order for the thermoluminescent dosimeters to have excellent performance, the dosimeters should have high sensitivity to measure radiation as low as possible and should show an optimal structure of thermally stimulated luminescence glow curves.
According to ICRP 60 [ICRP, 1990 Recommendations of the International Commission on Radiological Protection, ICRP Publication 60, Pergamon Press, Oxford, N.Y., 1990], radiation levels to be encountered by radiation workers should be as low as reasonably achievable (ALARA) and to measure low levels of radiation. thermoluminescent materials showing high sensitivity to even low doses are required.
Also, the thermally stimulated luminescence glow curve indicates luminescence intensity at different temperatures of thermal stimulation and helps in deciding the heating profile required to obtain a signal. Specifically, the area of the luminescence glow curve indicates the emission of light, which is proportional to the dose of radiation. Thus the area of the luminescence curve which is a standard mode for dose assessment is used as a signal for arriving at final doses through dose assessment algorisms including calibrations and various correction factors.
In thermoluminescence phenomena, some of the electrons produced by ionizing radiation remain trapped in the trapping level, and emit light when they receive thermal energy. Thus, electrons in shallow traps corresponding to luminescence peaks formed in a low-temperature range can be excited, even by thermal stimulation at room temperature alone without any external heating, and this phenomenon of exciting electrons at room temperature increases as the temperature of the luminescent glow peaks decreases. Accordingly, the TL from the low-temperature peaks causes some loss of the information on the passage of time at room temperature, after the thermoluminescent material is irradiated with radiation, thus reducing the reliability of assessment of cumulative exposure dose, which could be a the major advantage of TLD if the lower temperature peaks are dominant.
Therefore, in a preferred structure of thermally stimulated luminescence glow curves of thermoluminescent material, the main luminescence peaks formed in a high-temperature range should be intense and the luminescence peaks in a low-temperature range should be absent or negilible. Preferably the main peaks should be in the form of single peak having a simple structure with no peaks on either the lower temperature side or the higher temperature side of the main glow peak. The glow peaks at higher temperature side of the main peak necessitates heating to higher temperatures which affects the signal to nose ratio due to the enhanced incandescence light at higher temperatures and the trap distribution to cause a change in the sensitivity for the reuse of the TLD. The presence of the higher temperature peaks in the close vicinity of the main peaks, if not erased, result in higher residual signal which also affects the reusability.
Recently, studies have been actively conducted to develop thermoluminescent materials, which have high sensitivity even at sufficiently low radiation doses, and show thermally stimulated luminescence glow curves having single main peaks with a simple structure.
In the above-described thermoluminescence phenomena, the structure of thermally stimulated luminescence glow curves greatly changes according to the state of the trapping level in the forbidden band and depends on the kind and the concentration of dopants added and also on the type of thermal treatment in the synthesis of crystalline structure. Thus, in order to obtain the most preferred thermoluminescent material, it is required to find the right kind of dopants to be added and the optimal concentrations of dopants and the preparation procedure thereof.
These days, in the USA, China, Poland, France and the like, LiF thermoluminescent materials are actively being studied, and developed in the form of powder- or solid-type dosimeters depending on the intended use thereof.
The LiF-based thermoluminescent (TL) materials are widely used in the radiation dosimetry field, because they have various advantages in that they show low photon energy dependence in their responses and are near tissue-equivalent materials, meaning that their responses to photon radiation is similar to that of human tissue.
Particularly, LiF:Mg,Cu,P material consisting of LiF doped with Mg, Cu and P as activators was introduced first in the year 1978, and was commercialized in the middle of the 1980s as GR-200 in China and as MCP-N in Poland. This material has radiation sensitivity about 30 times as high as that of the previously widely used LiF:Mg,Ti material, and has recently received the most attention in the radiation dosimetry field.
However, this material has two major disadvantages in that it shows a rapid reduction in sensitivity when annealed at a temperature higher than 240° C., and in that it has a relatively high residual signal. The residual signal of the TL material is defined as the ratio of the second readout value to the first readout value and acts as an important obstacle when the TL dosimeter is repeatedly used. Recently, the Korea Atomic Energy Research Institute has conducted research to overcome such disadvantages, and, as a result, has developed a LiF:Mg,Cu,Na,Si material. This material was assessed for the residual signal, but did not show a great difference in residual signal compared to the prior material.